This invention relates generally to pipes for fluid transportation and particularly to a method of producing a multiple-wall pipe structure made up of two or more pipes of different diameters tightly fitted one within another, at least one of which is of a corrosion-resistant material. More particularly, the invention deals with lengths or sections of such dual- or multiple-wall pipe structure having their ends protected against the corrosive or oxidative action of ambient air or moisture during storage or shipment and of the fluid being transported in use.
Let us consider, as a typical example of the multiple-wall pipe structure under consideration, a dual-wall pipe intended for oil or petroleum recovery from its wells, for the conveyance of the recovered oil, or for use in nuclear power plants. The pipe for such applications must be pressure-, heat- and corrosion-resistant. Hence the dual-wall pipe structure takes the form of what is known as stainless-clad-steel, having an outer pipe, or pipe proper, of carbon steel and an inner pipe, or lining, of a stainless alloy commonly referred to as stainless steel. The carbon steel pipe and the stainless steel lining are manufactured separately and then nested together in tight fitting relationship by various methods familiar to the metal pipe specialists.
A corrosion problem arises with such stainless-steel-lined steel pipe because it is manufactured in unit lengths, or sections, to be joined in site as by welding or with use of couplings. During storage of the manufactured pipe lengths, particularly in open air, or during their shipment, corrosion or oxidation is liable to take place particularly at their unprotected ends. Even after installation, also, the corrosive fluid being conducted through the piping can easily attack the welded or coupled joints between the pipe lengths.
It has been suggested to protect the ends of multiple-wall pipe lengths against corrosion by, for example, "seal welding" or by the padding of weld metal. The "seal welding" technique is unsatisfactory, however, in the strength in the axial direction of the jointed pipe. The padding of weld metal, on the other hand, necessitates the application of considerable heat to the end portions of the pipe lengths, thus possibly straining or deforming them. Both of these welding techniques further make it difficult to machine the end portions of the pipe lengths into a state suitable for jointing them and particularly to cut external taper threads on the pipe end portions for connection by means of internally taper-threaded couplings.